1. Field of the Invention
The present invention is related to improvements in linear variable gain amplifiers. More particularly, the present invention relates to improvements in the operating range gain control of such amplifiers.
2. Background Art
Linear variable gain amplifiers are amplifiers which receive a pair of input signals and amplify the difference between the two signals in an output signal for subsequent use. Such amplifiers are used in many applications in common use today. For example, such devices are used in high-fidelity sound transmitting and receiving systems, as well as in servo systems such as are used for disk drive read channels.
It is desirable to control the gain of the linear variable amplifier while achieving a linear-in-dB gain control over a wide range of amplifier gains. Controlling the gain of the amplifier is accomplished typically through adjusting the bias current, as the voltage gain of the differential stage of the amplifier is directly related to the bias current.
Various systems have been proposed for increasing the gain of a linear variable gain amplifier, but the maximum bias current is limited by the lowest common mode voltage which does not have the transistors fall into the saturation region. However, as the supply voltage decreases, the usable bias level gets smaller. The prior art linear variable gain amplifier systems thus had a limited operating range, which is undesirable as it limits the usefulness of the amplifier.
Another limitation of the prior art linear variable gain amplifiers is the gain is often difficult to control and change quickly because the gain depends on the changing of current flowing through a transistor. It would be desirable to have an amplifier with discrete gain range control, but no such linear variable gain amplifier has been proposed in the prior art.
A third limitation of linear variable gain amplifiers is that low voltage applications frequently have an emitter-degenerated differential stage which makes it difficult to have a high-accuracy differential amplifier when operating at a low voltage. For applications which operate at low voltage, such as a servo control system, it would be desirable to have a high accuracy variable gain linear amplifier even when the input voltage is low.
Prior art linear variable gain amplifier systems have other limitations and disadvantages which will be apparent to those skilled in the art in view of the following description of the present invention.
The present invention overcomes the disadvantages and limitations of the prior art systems for controlling the operating range of a variable gain amplifier.
The present invention has the advantage that, in one embodiment, the linear variable gain amplifier system may obtain a high gain, with a corresponding high bias current, without driving an input transistor into saturation. Such a linear variable gain amplifier system allows the input transistors to remain in the active or linear region, rather than driving the transistors into a saturation level when the bias current goes high. Such a system allows a larger usable range of bias current levels in the linear variable gain amplifier system of the present invention.
A second advantage of the amplifier circuits of the present invention is to provide a greater gain range of the amplifier and a controlled or selected gain operating at a quick speed. In this way the same linear variable gain amplifier could be operating at one gain and switched quickly to operate at a different gain without traversing through intermediate gains through a ramping of current through a transistor operating in the linear range. In one embodiment of the present invention, an amplifier with a discrete gain range selection is provided through the use of selectable (binary or digital) control inputs. That is, a particular gain range can be selected from among several options and applied to the linear variable gain amplifier by selecting an effective resistance and switching that resistance into the amplifier circuit though the use of one or more transistors which are turned on. The same system can be operated at a different gain range at a different time by selecting a different control input, causing a different impedance and therefor a different gain to be seen by the linear variable gain amplifier system of the present invention. This gain range may be selected by applying a selected resistance across the emitter of the transistor, either individually or in selective combinations to adjust the effective resistance seen by the amplifier and thus adjust the gain of the amplifier.
A third advantage of the amplifier system of one embodiment of the present invention is a linear variable gain amplifier system suitable for low voltage applications. This system is a high-accuracy differential linear amplifier system which is particularly useful for an emitter-degenerated differential stage, allowing use of a high accuracy differential amplifier in low voltage applications (such as servo disk driver or in the IF (Intermediate frequency) or base-band amplifier for wireless receivers where the input electrical signal may be quite small and needs substantial amplification for subsequent use.
The present invention is a linear variable gain amplifier which has improved performance and enhanced operating range. The enhanced operating range originates from having a wider operating current in one embodiment and permits a lower voltage operation without degrading the performance in an other embodiment. In a third variation, the amplifier of the present invention allows for binary (or switch) selection of effective resistors, alone or in combination, to select a gain range for the amplifier.
The operating features of the present invention provide various improvements in the linear variable gain amplifier system and can be selected and combined as appropriate for a given application. Thus, one application may require both a low input and a wide operating range and another application may require selectable gain control along with either a wide operating range or a low voltage input.
Other objects and advantages of the several amplifier embodiments of the present invention will be apparent to those skilled in the relevant art in view of the following description of the preferred embodiment, taken together with the accompanying drawings and the appended claims.